Semi-submersible rigs and other vessels such as FPSOs are commonly used for the recovery of oil or gas, and are typically anchored to the seabed at the surface of the sea, above the oil or gas well, so that they can access the well through elongate tubing, risers or casing etc. Naturally, the floor of the rig heaves with the motion of the sea, whereas the casing or riser etc is fixed immovably to the wellhead on the seabed. This creates relative movement between the rig floor and the equipment inserted into the well, which is generally accommodated by heave compensators of known design.
When well intervention operations are carried out from such a vessel, the operator must insert the intervention tool into the outer casing at the rig floor, and then manipulate the tooling using the chosen delivery system (normally coiled tubing or wireline) towards the correct target. Since the intervention tool is connected between the heaving rig and the static well, the delivery system of coiled tubing or wireline delivery system is subject to cyclic movement, which is generally accommodated by a motion compensator on the derrick at the point where the wireline or coiled tubing is grasped and inserted into the well.
When the intervention tool delivery system is very long (such as coiled tubing or wireline delivery systems) the reel or other storage device for the elongate portion to be inserted into the well is generally stored on the rig floor, and problems have arisen when the reel of coiled tubing or wireline cable is moving cyclically relative to the compensated injector head.